Plasma deposition of amorphous semiconductors at microwave frequencies

ABSTRACT

Apparatus and method for plasma deposition of thin film photovoltaic materials at microwave frequencies. The apparatus avoids deposition on windows or other microwave transmission elements that couple microwave energy to deposition species. The apparatus includes a microwave applicator with conduits passing therethrough that carry deposition species. The applicator transfers microwave energy to the deposition species to transform them to a reactive state conducive to formation of a thin film material. The conduits physically isolate deposition species that would react to form a thin film material at the point of microwave power transfer. The deposition species are separately energized and swept away from the point of power transfer to prevent thin film deposition. The invention allows for the ultrafast formation of silicon-containing amorphous semiconductors that exhibit high mobility, low porosity, little or no Staebler-Wronski degradation, and low defect concentration.

FIELD OF INVENTION

The invention establishes a new realm of plasma chemistry and physicsthat enables the deposition of unique atomically-engineeredmulti-element compositions for photovoltaic applications that free theworld from its dependence on fossil fuels. More particularly, thisinvention solves the problem of depositing silicon-containingsemiconductors at high deposition rates to achieve highly efficientphotovoltaic materials with a low density of states that exhibit noStaebler-Wronski degradation. Most particularly, this invention relatesto plasma deposition of amorphous, nanocrystalline, microcrystalline,polycrystalline or single crystalline semiconductors at microwavefrequencies from multiple source gases, one of which includes fluorine,in a process that avoids undesirable coatings on microwave windows.

BACKGROUND OF THE INVENTION

Concern over the depletion and environmental impact of fossil fuels hasstimulated strong interest in the development of alternative energysources. Significant investments in areas such as batteries, fuel cells,hydrogen production and storage, biomass, wind power, algae, and solarenergy have been made as society seeks to develop new ways of creatingand storing energy in an economically-competitive andenvironmentally-benign fashion. The ultimate objective is to minimizesociety's reliance on fossil fuels and to avoid production of greenhousegases.

A number of experts have concluded that to avoid the seriousconsequences of global warming, it is necessary to maintain CO₂ atlevels of 350 ppm or less. To meet this target, based on currentprojections of world energy usage, the world will need 17 TW ofcarbon-free energy by the year 2050 and 33 TW by the year 2100. Theestimated contribution of various carbon-free sources toward the year2050 goal are summarized below:

Projected Energy Source Supply (TW) Wind 2-4 Tidal  2 Hydro    1.6Biofuels 5-7 Geothermal 2-4 Solar 600Based on the expected supply of energy from the available carbon-freesources, solar energy is clearly the most viable solution for reducinggreenhouse emissions and alleviating the effects of global climatechange. (See J. Esch, “Keeping the Energy Debate Clean How Do We Supplythe World's Energy Needs?”, IEEE Proc. 98(1), 39-41 (2010).)

Amorphous semiconductors are attractive materials for solar energyapplications. Among the amorphous semiconductors, amorphous silicon isknown to be a particularly promising solar energy material. Unlikecrystalline silicon, amorphous silicon is a direct gap material that hasstrong absorption over much of the solar spectrum. The strong absorptionmeans that high efficiency solar cells can be formed from thin layers ofamorphous silicon. As a result, solar panels based on amorphous silicon(or chemically-modified or structurally-modified forms of amorphoussilicon, including composite forms of amorphous silicon that includenanocrystalline phases) are lightweight, flexible, and readily adaptedto field use in a variety of installation environments.

S. R. Ovshinsky has long recognized the advantages of amorphous siliconand related materials as the active layer of solar cells and has beeninstrumental through his inventions and developments in advancingautomated and continuous manufacturing techniques for producing solarand photovoltaic devices based on amorphous semiconductors orcombinations of amorphous semiconductors with nanocrystalline,microcrystalline, polycrystalline or single crystalline semiconductors.Representative discoveries of S. R. Ovshinsky in the field of amorphoussemiconductors and photovoltaic materials include U.S. Pat. Nos.4,400,409 (describing a continuous manufacturing process for making thinfilm photovoltaic films and devices); 4,410,588 (describing an apparatusfor the continuous manufacturing of thin film photovoltaic solar cells);4,438,723 (describing an apparatus having multiple deposition chambersfor the continuous manufacturing of multilayer photovoltaic devices);4,217,374 (describing suitability of amorphous silicon and relatedmaterials as the active material in several semiconducting devices);4,226,898 (demonstration of solar cells having multiple layers,including n- and p-doped); 5,103,284 (deposition of nanocrystallinesilicon and demonstration of advantages thereof); and 5,324,553(microwave deposition of thin film photovoltaic materials).

Current efforts in thin film photovoltaic material manufacturing aredirected at increasing the deposition rate without impairingphotovoltaic efficiency and, in the case of silicon-containingmaterials, without exacerbating Staebler-Wronski degradation. Higherdeposition rates lower the cost of thin film solar cells and can lead toa dramatic decrease in the unit cost of electricity obtained from solarenergy. As the deposition rate increases, thin film photovoltaicmaterials become increasingly competitive with fossil fuels as a sourceof energy. Presently, PECVD (plasma-enhanced chemical vapor deposition)is the most cost-effective method for the commercial-scale manufacturingof amorphous silicon and related amorphous semiconductor photovoltaicmaterials. Current PECVD processes provide uniform coverage oflarge-area substrates with device quality photovoltaic material atdeposition rates of ˜1-5 {acute over (Å)}/s. This deposition rate,however, is insufficient to achieve cost parity with fossil fuels.

In order to enhance the economic competitiveness of plasma depositionprocesses, it is desirable to increase the deposition rate. Toeffectively compete with fossil fuels, it is believed that depositionrates of 100 Å/s or higher are needed. The deposition rate of prevailingplasma deposition techniques is limited by the high concentration ofintrinsic defects that develops in amorphous solar materials as thedeposition rate is increased. The intrinsic defects include structuraldefects such as dangling bonds, strained bonds, unpassivated surfacestates, non-tetrahedral bonding distortions, coordinatively unsaturatedatoms (e.g. two- or three-fold coordinated silicon or germanium). Thestructural defects create electronic states in the bandgap and near theband edges of amorphous semiconductors. The electronic states detractfrom solar conversion efficiency by (1) promoting nonradiativerecombination processes that deplete the concentration of free carriersgenerated by absorbed sunlight and (2) reducing hole mobility. Intrinsicdefects also contribute to degradation of the solar conversionefficiency of amorphous silicon and related materials through theStaebler-Wronski effect, an effect that leads to a 15-30% reduction inphotovoltaic efficiency with use over time.

S. R. Ovshinsky has demonstrated that the concentration of intrinsicdefects formed in a plasma-deposited material depends on thedistribution of species present in the plasma. A plasma is a complexstate of matter that includes ions, ion-radicals, neutral radicals andmolecules in multiple energetic states. In particular, S. R. Ovshinskyhas shown that certain charged species can be detrimental to the qualityof as-deposited amorphous semiconductors under conditions in which theypromote the creation of defects. Uncontrolled charged species tend tostrike the deposition surface with high kinetic energy and damage agrowing thin film material through bond cleavage. Bond cleavage createsdangling bonds and promotes the formation of locally strainedcoordination environments that may contribute to electronic defectstates. S. R. Ovshinsky has shown that neutral species in a plasma, incontrast, frequently promote more uniform bonding and lead to lowerdefect concentrations in as-deposited material. S. R. Ovshinsky hasultimately showed that the proper balance of charged and neutral speciesis essential to maximizing deposition rate and minimizing defects. Hehas further demonstrated that the optimal identity, concentration, andcharge of species in a plasma environment varies with plasma conditionsand can be constructively influenced through chemical modification withagents such as fluorine.

To minimize the concentration of intrinsic defects, current plasmadeposition processes are performed at low deposition rates. By slowingthe deposition process, the intrinsic defects that form in theas-deposited product material have the opportunity to equilibrate toenergetically-favored states that have more regular bondingconfigurations. As a result, the concentration of intrinsic defects isreduced. Unfortunately, the reduced deposition rate impairs the economiccompetitiveness of the process and prevents cost parity with fossilfuels.

In U.S. patent application Ser. Nos. 12/199,656; 12/209,699; and12/429,637; S. R. Ovshinsky described techniques for minimizing thedeleterious effect of uncontrolled charged plasma species on the defectconcentration. The patent applications describe techniques formaximizing the presence of neutral species and controlling the presenceand activity of charged species at the deposition surface throughpreferential formation of neutral species in the plasma activationprocess, magnetic confinement to regulate charged species, and/orseparation of undesirable charged species to form a charge-controlleddeposition medium. Through utilization of a charge-controlled depositionmedium, high quality amorphous or other silicon-containingsemiconductors can be formed at high deposition rates in a plasmadeposition process.

Another strategy for increasing the deposition rate of plasma-basedprocesses is to increase the plasma frequency. Conventional plasmadeposition processes are typically completed at radiofrequencies (e.g.13.56 MHz). As the plasma frequency is increased, the source gases usedin plasma deposition are activated more efficiently, more completely,and to higher energy states. Plasma excitation at microwave frequencies(e.g. 2.45 GHz), for example, leads to higher dissociation rates ofsource gases, generates higher fluxes of ions and neutrals, and createsa higher proportion of plasma species (ions, neutrals) sufficientlyenergetic to participate in the deposition process. The highdissociation rates and higher excitation energies associated withmicrowave plasmas improve process efficiency by providing much higherutilization of source gases than radiofrequency plasmas. The high fluxesand energies of ions and neutrals produced by microwave plasmas lead tosignificantly higher thin film deposition rates than radiofrequencyplasmas.

In addition to dissociation of a higher fraction of source gases, thehigh deposition rate accompanying microwave deposition of thin filmprecursors is also a consequence of the enhanced reactivity ofdeposition intermediates. Enhanced reactivity of depositionintermediates results from the higher energy of activation availablefrom microwave excitation. Microwave excitation produces depositionintermediates with higher internal energy by activating depositionprecursors to higher energy electronic and vibrational excited states.The higher internal energy makes the deposition intermediates lessstable and more conducive to the structural rearrangements and reactionson the deposition surface needed to form a thin film material.

Although enhanced reactivity of deposition precursors is beneficial fromthe standpoint of deposition rate, it oftentimes leads to unintendedside effects. A common problem in microwave deposition is the tendencyof reactive deposition intermediates to form thin films away from thesubstrate. Thin film coatings, for example, may develop on the interiorwalls of the deposition chamber and may serve as a source ofcontamination for subsequent depositions.

Since the deposition chamber is normally operated under vacuum or with acontrolled atmosphere, it has a limited volume and receives precursors,background gases, and energy from external sources. Materials aregenerally delivered by conduits through valves that pierce theboundaries of the chamber. Electrical energy (such as the bias betweenelectrodes needed to initiate a plasma or the resistive dissipation usedto heat a substrate) is typically supplied by wires that connect anexternal power source through the boundaries of the chamber to internalcomponents. The formation of thin film coatings on the openings oractuators of internal valves, or on internal components such aselectrodes or wires, may alter deposition conditions, impair theuniformity of deposition or prevent deposition altogether.

Unintended thin film coatings are particularly problematic when theyform on the windows of a deposition chamber through which theelectromagnetic energy used to activate a plasma from depositionprecursors is transmitted. In microwave deposition, for example, themicrowave generator is normally located remote from the depositionchamber. The generator produces microwaves and transmits them along amicrowave waveguide to the deposition chamber or a downstreamapplicator, where the microwaves pass through a window to energizedeposition intermediates or activate deposition precursors to formreactive species used to form a thin film material. To maximize themicrowave energy coupled to the deposition intermediates or precursors,it is necessary to insure that the window is highly transparent tomicrowave frequencies. If the reactive species generated by themicrowaves deposit the thin film material on the window and the thinfilm material absorbs microwaves, the transparency of the windowdecreases.

Decreased transparency of the window leads to two detrimental effects.First, any decrease in transparency leads to a reduction in themicrowave energy coupled to the deposition intermediates or precursors.Reduced microwave coupling means that the deposition species are lessdissociated, less energetic, less reactive, and as a result, thedeposition rate decreases. Second, continued exposure of amicrowave-absorptive thin film on the window to microwave radiationleads to localized heating of the thin film material that can causethermal stresses and potentially catastrophic failure of the window.

The detrimental consequences of thin film coatings on microwavetransmission windows do not arise if the coating is transparent tomicrowave radiation. Most dielectrics (including quartz, sapphire,diamond, boron carbide, SiO₂, and Si₃N₄) are highly transparent tomicrowave radiation and may be formed safely at high deposition rates ina microwave plasma process. Coatings made from lower bandgap materials(including metals and most semiconductors), however, are much lesstransparent to microwave radiation and present much more seriousconcerns over safety and process consistency. Many desirablephotovoltaic materials, including amorphous silicon andsilicon-germanium, absorb microwave radiation and are difficult tomanufacture in a microwave plasma process because the high reactivityconditions present in a microwave plasma promotes the formation ofundesirable coatings on the windows used to transmit microwave radiationto the deposition environment. Accordingly, there is a need for aprocess that permits microwave deposition of semiconducting photovoltaicmaterials.

SUMMARY OF THE INVENTION

The amount of energy absorbed by the Earth's atmosphere, oceans and landmasses in one hour is more than the amount of energy used by people onEarth in one year. This fact reveals that solar energy is the ultimatesolution to eliminating mankind's dependence on fossil fuels.Implementation of solar energy on a scale sufficient to meaningfullyreduce fossil fuel consumption has been hindered, however, by economicsand concerns about cost. Dr. Steven Chu, winner of a Nobel Prize inphysics and presently the Secretary of Energy, summed up the problem ina New York Times article that appeared on Feb. 12, 2009, by stating thata revolution in science and technology would be needed if the world isto reduce its dependence on fossil fuels and curb the emissions ofcarbon dioxide and other heat-trapping gases linked to global warming.Dr. Chu also stated that a five-fold improvement in solar technology wasneeded to adequately address global warming and reduce the world'sdependence on fossil fuels. This invention can be summarized most simplyas providing the revolution and improvement in solar technology that Dr.Chu referred to.

With the invention, the unit cost of solar energy is at or below thecost of fossil fuels. As a result, widespread implementation of theinstant invention will allow mankind to reduce its dependence on fossilfuels and serves the higher goal of democratizing energy by enabling allcountries, regardless of natural resources, to become self-sufficient inenergy. Concerns over the scarcity of fossil fuels, conflicts oversources of fossil fuel will be eliminated, and national and worldwidesecurity will be enhanced.

The invention is predicated on a fundamental advance in plasma chemistryand physics that allows for a tremendous increase in the throughput anddeposition rate of photovoltaic materials containing Group IV elements(e.g. Si, Ge, Sn) in a continuous manufacturing process. The fundamentaladvance in plasma chemistry and physics enables a unique atomicengineering of multi-element compositions that affords a method ofcontrolling and forming thin film photovoltaic materials in the presenceof a microwave plasma. With the invention, the deposition rate of thinfilm photovoltaic materials based on silicon can be dramaticallyincreased for the first time without introducing the defects, thedensity of states and Staebler-Wronski degradation that have heretoforediminished photovoltaic efficiency and frustrated efforts to achievecost parity with fossil fuels.

The invention enables for the first time a gigawatt or more ofmanufacturing capacity in a single machine of a size that fits within anordinary manufacturing plant. Because of this invention, it will nolonger be necessary to run multiple manufacturing processes in multiplelocations in parallel or to build multiple machines in series to realizeoutput on the gigawatt scale. The tremendous cost reduction afforded bythis invention will motivate the development of new industries that willprovide high-valued jobs that stimulate the economy and promote theeducational system.

The foregoing benefits of the instant invention are more particularlyrealized in the exemplary embodiments now summarized:

This invention provides a method and apparatus for the microwavedeposition of atomically-engineered, multi-element, silicon-containingphotovoltaic materials with unique chemical bonding and structuralconfigurations, resulting in new physics. The invention provides thehigh deposition rate advantage associated with microwave deposition,while avoiding or minimizing the problems of (1) forming unintendedcoatings in the deposition chamber or on the windows used to transmitmicrowave radiation; (2) creating electronic defect states in thebandgap that detract from photovoltaic efficiency; and (3) degradationof photovoltaic efficiency over time upon continuous exposure of thematerial to incident radiation during operation due to theStaebler-Wronski effect.

The silicon-containing photovoltaic material provided by the instantinvention is a thin film material that can be formed at high speedswithout compromising the quality of the material. The invention solvesthe heretofore insurmountable problem of realizing the benefit of highrate deposition from microwave plasma excitation without creating a highconcentration of structural or electronic defects that produce a highdensity of states in the bandgap. The thin film material of the instantinvention features new bonding relationships that provide a lowconcentration of defects, a low density of states, a dense, non-porousstructure, and little or no Staebler-Wronski degradation. The instantinvention constitutes the advent of a new regime of depositionconditions that exploits fundamentally new physics and chemistry toachieve superior performance of photovoltaic materials based on silicon(and other fourfold coordinate elements) in an ultrafast depositionprocess. Silicon-based materials available from the instant inventioninclude materials that have an amorphous, nanocrystalline,microcrystalline, polycrystalline, or single crystalline structure aswell as materials that combine two or more of such structures.

This invention reduces or eliminates the Staebler-Wronski effect byusing fluorine in a microwave plasma to engineer the depositionenvironment to insure formation of silicon-containing photovoltaicmaterials with improved bonding and unique structural configurations.Fluorine is active not only in the plasma, but also within and on thesurface of the product material. The beneficial effect of fluorineenables the deposition of silicon-containing photovoltaic materials witha low density of states and essentially no Staebler-Wronski degradationat heretofore unattainable rates.

The apparatus generally includes a microwave generator, microwavewaveguide, microwave applicator, and deposition chamber. The microwavegenerator produces microwaves and launches them down the waveguidetoward the applicator. The applicator couples the microwaves todeposition species flowing through one or more conduits that passthrough the applicator. The conduits are formed from a material thattransmits microwave radiation to permit coupling of microwave energy tothe deposition species. The deposition species may be neutral precursorsin a ground or excited energetic state, ionized precursors, freeradicals formed from a neutral precursor, or constituents of a plasma.Coupling of microwave energy to the deposition species energizes them topromote reactivity and increase deposition rate. The energizeddeposition species exit the one or more conduits, enter the depositionchamber, and form a thin film material on a substrate. The process mayfurther include the introduction of one or more supplemental materialstreams to the deposition chamber that have not been subjected tomicrowave excitation. The supplemental material streams may combine withthe energized species exiting the one or more conduits to provide adeposition medium from which a thin film material is formed.

The thin film material is generally a semiconductor or amorphoussemiconductor material. The thin film material typically includessilicon and/or germanium and may be an intrinsic semiconductor or asemiconductor doped n-type or p-type. Embodiments include silicon,germanium, and alloys of silicon and germanium in amorphous,nanocrystalline, microcrystalline and/or polycrystalline forms. Thematerials also include hydrogenated and/or fluorinated variants.

Deposition species include silane, fluorosilanes, germane,fluorogermanes, and mixtures thereof. Deposition species may alsoinclude treatment gases that passivate or modify the surface of the thinfilm material. Treatment gases may or may not provide elements that areincorporated into the thin film material. Treatment gases includehydrogen, hydrogen fluoride, fluorine, and noble gases. Carrier gasessuch as argon, neon or helium may also be combined with one or moredeposition species or treatment gases in a conduit of the applicator.

The presence of fluorine in the microwave deposition environment(whether from a deposition precursor, treatment gas, or supplementalmaterial stream) is believed to facilitate new structural organizationsof the multiple elements present in the environment at or adjacent tothe deposition surface. The new structural organizations are a new formof atomic engineering that enables the high speed formation ofsilicon-containing photovoltaic materials in a bonding configurationthat avoids defects, improves photovoltaic efficiency, and preventsStaebler-Wronski degradation. The effective amount of fluorineincorporated into the product film ranges from 0.1% up to the etchingthreshold of fluorine. The etching threshold of fluorine corresponds tothe concentration of fluorine at which detrimental etching of theproduct film begins. The etching threshold of fluorine depends on thecharacteristics of the deposition environment, including theconcentration of hydrogen. In one embodiment, the etching threshold offluorine is about 7%.

The conduits of the microwave applicator are transparent to microwaveradiation and are formed from a dielectric material, such as an oxide ornitride. Representative dielectric materials include SiO₂, quartz,Al₂O₃, sapphire, transition metal oxides, silicon nitride, and aluminumnitride.

In one embodiment, the applicator includes two or more conduits, each ofwhich carries a different deposition species. The conduits may bephysically separated or one conduit may be concentric with or otherwisehoused within another. The conduits may be oriented in a directionaligned or non-aligned with the direction of microwave propagation inthe applicator. In one embodiment, the conduits are orientedperpendicular to the direction of microwave propagation.

One or more deposition species and/or carrier gases enter each of theconduits of the applicator and are energized by microwave radiation. Inone embodiment, the deposition species or carrier gases in each of twoor more conduits are energized with a common source of microwaveradiation. In another embodiment, separate sources of microwaveradiation are used to energize deposition species or carrier gases intwo or more conduits. The energized deposition species and/or carriergases exit the conduits and enter a deposition chamber. In thedeposition chamber, the energized deposition species are directed to asubstrate and a thin film material is formed. The substrate may bestationary or mobile. In one embodiment, the substrate is a continuousweb.

The deposition chamber may further include one or more injection portsfor delivering supplemental material streams to the substrate. Thesupplemental material streams may include precursors, intermediates,treatment gases, background gases or carrier gases. The supplementalmaterial streams may combine with the energized deposition mediumentering the chamber in the vicinity of the substrate and mayparticipate in the deposition process by reacting or interacting withthe energized deposition medium to influence the composition orcharacteristics of the deposited thin film material.

The deposition chamber may also include a supplemental energizing sourceto prevent or slow relaxation or decay of the energized species enteringthe deposition chamber from one or more conduits of the microwaveapplicator. The supplemental energy source can also be used to activatespecies in the deposition chamber to form intermediate species throughbond cleavage. The presence of fluorine, for example, can be increasedby activating SiF₄ to cleave an Si—F bond to liberate fluorine. In oneembodiment, the supplemental energy source is an electromagnetic sourcethat includes an antenna array. The antenna array may generate orsustain an electromagnetic field and may further provide phase control.In one embodiment, the electromagnetic field is a microwave field.

The deposition chamber is further equipped with means to meter, monitor,modulate and calibrate the presence and distribution of species in thegrowth ambient. This capability permits fine control over the ratios ofthe multiple elements in deposition environment to insure optimumconditions for high speed deposition. The relative amounts of fluorineand hydrogen are particularly important to the success of the invention.It is desirable to maximize the concentration of fluorine in the productfilm, but the presence of too much fluorine in the growth ambientpromotes an undesirable etching effect that increases the porosity ofthe product film. The presence of hydrogen in the product film can aidin passivating defects, but too much hydrogen promotes theStaebler-Wronski effect. In addition, fluorine and hydrogen can interactwith each other to deplete the concentration of fluorine and/or hydrogenavailable to assist the process of depositing the product film or tobecome incorporated into the product film. Proper control of the ratioof fluorine to hydrogen is important to realizing the superiorphotovoltaic materials available from this invention. In one embodiment,it is desirable to maximize the presence of fluorine in an energizedstate and to minimize the presence of hydrogen in an energized state.

The deposition chamber may be interconnected to one or more additionaldeposition or processing units. Additional deposition units permit theformation of multilayer thin film structures that include materials ofdifferent composition. Thin film structures include p-n junctions, p-i-nstructures, tandem cells, or triple junction cells, where at least onelayer is formed according to the instant invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 depicts a system for the microwave deposition of thin filmmaterials.

FIG. 2 depicts in side view an embodiment of a microwave applicator withconduits delivering different deposition species.

FIGS. 3A-3E depict in top view embodiments of a microwave applicatorwith conduits delivering different deposition species.

FIG. 4 depicts a system for the microwave deposition of thin filmmaterials that includes at least one energized deposition medium streamand at least one non-energized deposition medium stream.

FIG. 5 presents a compositional analysis of a silicon-containingphotovoltaic material in accordance with the instant invention.

FIG. 6 presents a compositional analysis of a silicon-containingphotovoltaic material in accordance with the instant invention.

FIG. 7 shows the optical absorption spectrum of the silicon-containingphotovoltaic material with the composition depicted in FIG. 5.

FIG. 8 shows optical absorption spectrum of the silicon-containingphotovoltaic material with the composition depicted in FIG. 6.

FIG. 9 shows the evolution of the optical absorption spectrum of thesilicon-containing photovoltaic material with the composition depictedin FIG. 5 upon exposure to solar radiation.

FIG. 10 compares the effect of solar radiation on conventional amorphoussilicon and an amorphous silicon material in accordance with the instantinvention.

FIG. 11 shows the dependence of the μτ product on the ratio Si₂H₆/SiF₄for a series of samples.

FIG. 12 shows the dependence of deposition rate on the ratio Si₂H₆/SiF₄for a series of samples.

FIG. 13 shows the dependence of the μτ product on substrate temperatureat a fixed Si₂H₆/SiF₄ ratio for a series of samples.

FIG. 14 depicts a portion of a deposition system that includes a movingcontinuous web substrate.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Although this invention will be described in terms of certain preferredembodiments, other embodiments that are apparent to those of ordinaryskill in the art, including embodiments that do not provide all of thebenefits and features set forth herein and including embodiments thatprovide positive benefits for high-volume manufacturing, are also withinthe scope of this invention. Accordingly, the scope of the invention isdefined only by reference to the appended claims.

The instant invention provides an apparatus and method for microwaveplasma deposition of thin film materials. The invention is especiallysuited for the microwave plasma deposition of materials that absorbmicrowave radiation. A schematic of a microwave deposition system isdepicted in FIG. 1. System 100 includes microwave generator 105 thatcreates a field of microwave radiation and launches it through microwavewaveguide 110 to microwave applicator 115. Microwave generator 105typically includes a magnetron and delivers a field of microwaveradiation at a single frequency (e.g. 915 MHz, 2.45 GHz, 5.8 GHz).Applicator 115 couples the microwave radiation to deposition speciespassing through conduits 120 and 125. Conduit 120 receives one or moredeposition species in stream 130 from source 140 and conduit 125receives one or more deposition species in stream 135 from source 145.The microwave radiation couples to deposition species provided toconduits 120 and 125 to produce streams 150 and 155, respectively,containing energized deposition species that are delivered to depositionchamber 160 for formation of a thin film.

Although not shown, the deposition system may further include anisolator directly after microwave generator 105 to protect it fromback-reflected microwave radiation. The isolator includes a circulatorand a dummy load to neutralize back-reflected microwaves. The depositionsystem may also include a directional coupler in the waveguide run todetect and monitor forward and reflected microwave power, and a tuner tomatch the impedance of the load with the impedance of the source.Adjustment of the tuner minimizes the reflected power level. Atermination device or sliding short circuit may also be connected to thedownstream end of the applicator to assist with impedance matching or toestablish a standing wave condition that maximizes microwave power inthe vicinity of the conduits to increase the transfer of microwave powerto the deposition species.

Applicator 115 may include two or more conduits for deliveringdeposition species to a region of microwave coupling (power transfer).The conduits provide for physical separation of two or more streamscontaining deposition precursors, while permitting simultaneousmicrowave excitation of the individual streams. The conduits receivedeposition species from a source and transport them to an interiorcavity of the applicator for coupling to the microwave radiationprovided by waveguide 110. The coupling transfers energy from themicrowave radiation to the deposition species to activate or otherwiseenergize them to a high energy state. The energized deposition speciesare then delivered by the conduits to the deposition chamber fordeposition of a thin film material.

The high energy state created by transfer of microwave power is areactive state and enhances reactions between deposition species. Therates of reactions between deposition species that occur in anon-energized state are generally increased when the deposition speciesare placed in an energized state and reactions that do not otherwiseoccur between deposition species may be induced in the energized state.Physical separation of the deposition species by the conduits providesthe benefit of preventing reactions between deposition species in theregion where microwave power (or energy) is transferred to thedeposition species. As a result, the formation of thin film materials inthe region of power transfer is avoided.

FIG. 2 shows an enlargement of applicator 115 in side view. Microwaveradiation from waveguide 110 enters cavity 117, which couples microwaveradiation to the deposition species in streams 130 and 135. Inparticular, cavity 117 is configured to transfer microwave power orenergy to the deposition species in streams 130 and 135 in regions 132and 142, respectively. Regions 132 and 142 correspond to the regions oftransfer of microwave power (or energy) to streams 130 and 135,respectively, and coincide with the interior portions of conduits 120and 125, respectively, that pass through the interior of applicator 115.

Microwave transfer region 132 includes boundary or window 134 thattransmits microwave radiation through conduit 120 to deposition speciesin stream 130. Microwave transfer region 142 includes boundary or window144 that transmits microwave radiation through conduit 125 to depositionspecies in stream 135. Transfer of microwave power (or energy) todeposition species in stream 130 produces energized deposition speciesthat exit applicator 115 in stream 150. Transfer of microwave power (orenergy) to deposition species in stream 135 produces energizeddeposition species that exit applicator 115 in stream 155.

To increase the deposition rate of a thin film material in depositionchamber 160, it is desirable to maximize the transfer of microwave power(or energy) to the deposition species transported through conduits 120and 125. Greater transfer of microwave power (or energy) leads to morecomplete excitation or activation of the deposition species, a higheroverall energy for the deposition species, greater dissociation, andgreater reactivity. In one embodiment, the efficiency of transfer ofmicrowave power (or energy) to the deposition species is increased byforming a standing wave of microwave radiation in cavity 117 ofapplicator 115 and locating one or both of conduits 120 and 125 so thatdeposition streams 130 and/or 135 pass through a region of maximum orlocally maximum intensity of the standing wave pattern. A standing wavecan be formed from the field of microwave radiation that enters cavity117 by adjusting the length of the cavity or by attaching a terminationdevice or sliding short to the cavity.

Physical separation of streams 130 and 135 in the region of microwavepower (or energy) transfer prevents reactions between energizeddeposition species in stream 150 and energized deposition species instream 155 that might otherwise occur to form a coating on the conduitwindows. By delaying the interaction of the energized deposition speciesin streams 150 and 155 until after delivery into deposition chamber 160,the formation of a thin film material occurs away from the region ofmicrowave power (or energy) transfer and the coating of conduit windowsis avoided.

FIG. 3A depicts cavity 117, conduits 120 and 125, and deposition streams130 and 135 as shown in FIG. 1 in top view. In the embodiment of FIG.3A, conduits 120 and 125 have a generally circular cross-section. Inother embodiments, the cross-section of the conduits may have anothercross-sectional shape, including elliptical, oval, square, rectangular,polygonal, or other closed contour. The cross-sectional shape and/ordimensions may also differ for the different conduits introduced intocavity 117 of applicator 115. In the embodiment of FIG. 3A, conduits 120and 125 are generally aligned in the direction of microwave propagation.In other embodiments, the positions of conduits 120 and 125 may benon-aligned in the direction of microwave propagation. FIG. 3B, forexample, illustrates an embodiment in which conduits 120 and 125 arealigned in a direction generally orthogonal to the direction ofmicrowave propagation. The scope of the invention extends to arbitraryplacement or orientation of two or more conduits relative to thedirection of microwave propagation.

In the embodiments of FIGS. 3A and 3B, conduits 120 and 125 arephysically displaced from each other and the cross-sections of conduits120 and 125 are non-overlapping. The objective of maintaining a physicalseparation between two or more streams containing different depositionspecies may also be achieved with conduits having overlappingcross-sections. Two or more conduits may, for example, be concentricwith each other or one or more conduits may be otherwise housed withinanother conduit. FIG. 3C depicts an embodiment in which conduits 120 and125 are concentric (co-axial) with each other within applicator 115 andFIG. 3D depicts an embodiment in which conduit 120 is housed within, butnot concentric with conduit 125. In FIGS. 3C and 3D, the boundary ofconduit 120 prevents intermixing of deposition species flowing inconduits 120 and 125. Deposition stream 130 is delivered to the interiorof conduit 120 and deposition stream 135 is delivered to the portion ofthe interior of conduit 125 that is exterior to conduit 120. In theembodiment of FIG. 3C, for example, deposition stream 135 experiences agenerally annular flow as it passes through applicator 115. Since theboundaries of conduits 120 and 125 transmit microwave radiation,microwave power (energy) can be transferred to each of depositionstreams 130 and 135 in the embodiments of FIGS. 3C and 3D. FIG. 3E showsan embodiment in which conduits 120 and 125 are housed within conduit126. In the embodiment of FIG. 3E, physical separation is maintainedbetween deposition streams 130, 135, and 136 within the interior ofapplicator 115.

The number, size, shape, and relative positioning of two or moreconduits permits control over the electromagnetic field provided by themicrowave radiation. The relative intensity, for example, of themicrowave field varies spatially and conduits carrying particulardeposition species or precursors can be positioned in regions of high orlow electromagnetic intensity. This flexibility affords a degree ofcontrol over the relative reactivity of multiple deposition precursorsand assists the objective of engineering, on an atomic scale, theinteraction and spatial distribution of the multiple elements that makeup the thin film materials of the instant invention.

Conduits 120 and 125 are formed from a material that transmits microwaveradiation. Preferably, conduits 120 and 125 are highly transparent tomicrowave radiation. Dielectric materials, such as oxides and nitrides,are among the dielectric materials that may be used to form conduits 120and 125. Representative dielectric materials include SiO₂, quartz,Al₂O₃, sapphire, transition metal oxides, silicon nitride, aluminumnitride, and transition metal nitrides.

The embodiment shown in FIG. 1 includes an applicator having twoconduits for delivery of streams of deposition species. In theembodiment of FIG. 1, the deposition species in the two conduits areenergized or activated by a common field of microwave radiation. Theinstant invention extends generally to applicators including two or moreconduits for delivering two or more independent streams of depositionprecursors, where the independent streams are energized or activated bya common field of microwave radiation. The instant invention alsoincludes embodiments in which two or more streams of depositionprecursors are provided to two or more applicators, each of whichincludes one or more conduits. In these embodiments, a separatemicrowave generator may be used for each applicator to achieveindependent control over the frequency and/or power of the field ofmicrowave radiation used to energize or activate different streams ofdeposition precursors.

FIG. 4 depicts an embodiment in which supplemental material streams aredirected to the deposition chamber in combination with an energizeddeposition medium. System 165 includes microwave applicator 166 thatreceives input stream 168 and energizes it with microwave radiation asdescribed hereinabove to form energized deposition medium 170 that isdelivered to deposition chamber 172. As described hereinabove, inputstream 168 may include one or more components, where each component is adeposition precursor, intermediate, carrier gas, or diluent gas. System165 further includes inlets 174 and 176 that deliver supplementalmaterial streams 178 and 180 to deposition chamber 172. Supplementalmaterial streams 178 and 180 may be precursors, intermediates, carriergases, diluent gases, or background gases and are directly delivered todeposition chamber 172 without being activated or energized in microwaveapplicator 166. Supplemental material streams 178 and 180 combine withenergized deposition medium 170 in the vicinity of substrate 182positioned on mount 184. Supplemental material streams 178 and 180interact, dilute, or react with energized deposition medium 170 at or onsubstrate 182 to form thin film material 186.

The embodiment shown in FIG. 4 depicts a deposition system that includestwo supplemental material streams in combination with a microwaveapplicator that provides a single energized deposition medium stream. Inrelated embodiments, the microwave applicator may provide two or moreenergized deposition medium streams or two or more microwaveapplicators, each of which provides one or more energized depositionmedium streams may be employed. The number of supplemental streams maybe one or more. In one embodiment, the one or more supplemental materialstreams are electrically neutral. In another embodiment, the one or moresupplemental material streams are non-ionized. In a further embodiment,the one or more supplemental material streams are in an electronicground state.

The deposition chamber may also include an internal energizing source toprevent or slow relaxation or decay of the energized species enteringthe deposition chamber from the one or more conduits of the one or moremicrowave applicators. As noted hereinabove, microwave excitation ofmaterial streams passing through a conduit of a microwave applicator mayenergize or ignite a plasma therefrom. As the energized or ignitedmaterial stream flows away from the region of microwave coupling andenters the deposition chamber, it may no longer be influenced or excitedby the microwave field present in the applicator. As a result, theexcited, energized, activated or ignited species in the energizeddeposition medium delivered to the deposition chamber may relax or decayto lower energy states. When relaxation or decay occurs, thedistribution of species present may be altered and the characteristicsof the ultimate thin film may be compromised due, for example, to ahigher prevalence of defects or impurities. The extent of relaxation ordecay depends on the separation between the substrate and point of entryof the energized deposition medium into the deposition chamber,frequency of collisions between species within the energized depositionmedium and the intrinsic decay rates of the individual species presentin the energized deposition medium.

In one embodiment, the supplemental energy source is an electromagneticsource that includes an antenna array. The antenna array may generate orsustain an electromagnetic field and may further provide phase control.In one embodiment, the electromagnetic field is a microwave field.

The deposition species that may be introduced into the conduits of amicrowave applicator or inlets providing supplemental material streamsinclude neutral gases, ionized gases, pre-energized gases, plasmas, orcombination thereof. The instant invention provides a particular benefitfor combinations of deposition species that are capable of reacting (ineither an energized or non-energized state) to form a thin film materialcapable of absorbing microwave radiation. Deposition species generallyinclude gas phase materials that contain silicon, germanium, tin,fluorine, and/or hydrogen. Representative deposition species includesilane (SiH₄), disilane (Si₂H₆), fluorinated forms of silane (SiF₄,SiF₃H, SiF₂H₂, SiFH₃), germane (GeH₄), fluorinated forms of germane(GeF₄, GeF₃H, GeF₂H₂, GeFH₃), as well as ionized, energized, oractivated forms thereof, and combinations thereof. Deposition speciesalso include hydrogen gas, fluorine gas, NF₃ gas, and hydrogen fluoridegas, as well as carrier or background gases such as argon, helium,krypton, or neon.

Deposition species may or may not contribute an element to the intendedthin film composition. A deposition species may, for example, act as asurface treatment agent that improves the quality of the deposited film.A fluorinated gas, for example, may function as a surface treatment gasto passivate defects or saturate dangling bonds at the surface of thedeposited thin film material. Alternatively, a deposition species mayfacilitate initiation of a plasma or assist in establishing a particulardeposition pressure even though it does not contribute an element to thedeposited material.

Fluorinated deposition species are advantageous because fluorinepromotes regular tetrahedral coordination of silicon, germanium and tinin thin film materials, relieves bond strain, acts to passivate danglingbonds and other defects that produce tail states or midgap states thatcompromise carrier mobility in photovoltaic materials, and assists inthe formation of nanocrystalline, intermediate range order, ormicrocrystalline phases of silicon and germanium. (For more informationsee, for example, the following references by S. R. Ovshinsky: U.S. Pat.No. 5,103,284 (formation of nanocrystalline silicon from SiH₄ and SiF₄);U.S. Pat. No. 4,605,941 (showing substantial reduction in defect statesin amorphous silicon prepared in presence of fluorine); and U.S. Pat.No. 4,839,312 (presents several fluorine-based precursors for thedeposition of amorphous and nanocrystalline silicon); the disclosures ofwhich are incorporated by reference herein).

Silane (SiH₄) has been widely used as a deposition precursor foramorphous silicon, but is known to produce material that has poorelectronic properties due to the presence of a particularly highconcentration of dangling and strained bonds. Deposition of amorphoussilicon from silane in the presence of high hydrogen (H₂) dilution hasbeen shown to improve the electronic properties of amorphous silicon.Inclusion of excess hydrogen in the deposition process has the effect ofpassivating dangling bonds and relieving bond strain to provide amaterial with a lower concentration of defects, a lower density ofstates, and better carrier transport properties. Hydrogen dilutionprovides benefits similar to fluorine, but has the drawback of promotinga time-dependent degradation of photovoltaic efficiency through theStaebler-Wronski effect when present above a certain concentration.

To date, efforts to increase the plasma deposition rate of amorphoussilicon from silane (alone or in the presence of hydrogen dilution) byincreasing the plasma frequency from the radiofrequency range to themicrowave frequency range have been frustrated by an enhancement in theproduction of solid phase, particulate silanaceous byproducts that hasbeen observed as the plasma frequency is increased. The silanaceousbyproducts are thought to be long chain or polymeric compounds ofsilicon and hydrogen (e.g. polysilanes) and deposit throughout thedeposition chamber, including on the windows used to couple microwaveenergy to silane and/or hydrogen. Since the silanaceous byproductsabsorb microwave radiation, microwave deposition of silane underconditions of high hydrogen dilution has proved to be a commerciallyimpractical process. In addition, the presence of silanaceous byproductsis thought to contribute to Staebler-Wronski degradation. As a result,the benefits of high hydrogen dilution have been commercially realizedonly in radiofrequency plasma processes to avoid production of excesshydrogen and suppress formation of silanecous byproducts and thedeposition rates have been accordingly low.

In one embodiment of the instant invention, materials with propertiescomparable or superior to those available from a high hydrogen dilutionprocess are realized in a high rate microwave plasma deposition processby delivering SiF₄ to one conduit of a microwave applicator and H₂ to asecond conduit of the same or separate applicator. Separation of thesource of silicon from the source of hydrogen enables deposition ofamorphous, intermediate range order, nanocrystalline, andmicrocrystalline forms of silicon in a microwave plasma process. SinceSiF₄ is free from hydrogen, its excitation or activation by microwaveradiation does not lead to the production of polysilane or relatedbyproducts. As a result, the formation of unintended silanaceouscoatings on the microwave window is avoided and the severity ofStaebler-Wronski degradation is significantly reduced. Similarly,microwave activation or excitation of hydrogen in the absence of siliconoccurs without the production of undesirable solid phase byproducts. Asa result, the distribution of species needed to form high qualitysilicon-based photovoltaic materials can be created in a continuousprocess without corrupting the microwave windows. The species may thenbe transported away from the region of microwave coupling and combinedin the vicinity of a substrate for deposition of a thin film material.

The relative amounts of SiF₄ and H₂ may be adjusted by controlling thepressure or flow rate of each in their respective conduits or bycombining either or both of SiF₄ and H₂ with a carrier or backgroundgas. Inclusion of deposition species such as F₂ or HF provide furthercontrol over the relative amounts of silicon, hydrogen and fluorinepresent in the deposition environment established in the vicinity of thesubstrate. Adjustment of the relative amounts of deposition speciescontaining silicon, germanium, hydrogen, and/or fluorine permits controlover the degree of crystallinity and microstructure of the thin filmmaterial deposited on the substrate as well as control over the densityof states and severity of the Staebler-Wronski effect.

In other embodiments, a fluorine-containing gas and ahydrogen-containing gas may be delivered by separate conduits of one ormore microwave applicators. SiF₄ and SiH₄, for example, may be deliveredby separate conduits of a microwave applicator. Similarly, SiH₄ and afluorine-containing gas (e.g. F₂, NF₃, or a fluorinated germanium gas)may be delivered by separate conduits of one or more microwaveapplicators.

In still other embodiments, one or more of a silicon-containing gas,germanium-containing gas, fluorine-containing gas, orhydrogen-containing gas may be delivered by a microwave applicator as anenergized deposition medium to a deposition chamber and others of asilicon-containing gas, germanium-containing gas, fluorine-containinggas, or hydrogen-containing gas may be delivered as non-energizedsupplemental material streams to the deposition chamber. For example,one or more of SiF₄, SiH₄, H₂, or F₂ may be delivered by a microwaveapplicator as an energized deposition medium to a deposition chamber andothers of SiF₄, SiH₄, H₂, or F₂ may be delivered as non-energizedsupplemental material streams to the deposition chamber.

In one embodiment, SiF₄ is activated by microwave energy in a conduit ofan applicator and delivered to a deposition equipped to provide H₂ as asupplemental material stream, where the H₂ stream has not been activatedby microwave radiation. In another embodiment, SiF₄ is activated bymicrowave energy in a conduit of an applicator and delivered to adeposition equipped to provide SiH₄ as a supplemental material stream,where the SiH₄ stream has not been activated by microwave radiation. Instill another embodiment, fluorine is provided both in a supplemental,non-energized material stream and as a microwave-energized materialstream from an applicator. The supplemental fluorine stream may includeF₂ or HF diluted by a carrier gas such as a noble gas.

The instant invention further contemplates delivery of precursors inseparate streams that are activated by electromagnetic radiation atdifferent frequencies. As noted hereinabove, microwave activation ofsilane is believed to produce materials with a particularly pronounceddegree of Staebler-Wronski degradation because of the high concentrationof active hydrogen released from silane. A lesser degree of activehydrogen is formed, however, upon excitation of silane by aradiofrequency electromagnetic field. In one embodiment of the instantinvention, the deposition process includes microwave activation of SiF₄and radiofrequency activation of silane. Radiofrequency activation ofsilane provides a controlled source of hydrogen that allows formanagement of the hydrogen-to-fluorine ratio in the depositionenvironment.

The structural and compositional control afforded by the instantinvention further provides silicon-containing semiconductors, includingamorphous semiconductors, that exhibit little or no Staebler-Wronskidegradation. One of the drawbacks associated with utilizing highhydrogen dilution in forming amorphous silicon is a degradation ofphotovoltaic efficiency over time known as the Staebler-Wronski effect.Although high hydrogen dilution conditions form amorphous siliconmaterials with improved photovoltaic efficiency, the effect is notpermanent and photovoltaic efficiency gradually decays over time withpersistent exposure to solar energy. The origin of the Staebler-Wronskieffect is not fully understood, but is believed to involve aphotogeneration of electronic defect states or carrier trapping centersby incident sunlight. The degradation effect has been observed to becomemore severe as the extent of hydrogen dilution increases.

A pronounced Staebler-Wronski effect is one reason why attempts in theprior art to prepare amorphous silicon in a microwave deposition processhave been unsuccessful. Although microwave conditions have provided highdeposition rates in the prior art, the resulting amorphous siliconmaterial has suffered from an unacceptably high degree ofStaebler-Wronski degradation. It is believed that the more energeticconditions associated with microwave plasmas (relative to radiofrequencyplasmas) releases too much hydrogen from the silane (SiH₄) precursor toprovide an especially high degree of hydrogen dilution and an especiallysevere Staebler-Wronski effect.

The instant inventor believes that inclusion of fluorine in thecomposition of silicon-containing amorphous semiconductors can remedythe Staebler-Wronski effect by strengthening bonds and improving thestructural integrity of the material to render it less susceptible tolight-induced defect creation. The instant inventor recognizes, however,that direct inclusion of fluorine in a prior art plasma depositionprocess (at microwave or radiofrequency frequencies) leads to a furthercomplication. Specifically, microwave activation of afluorine-containing precursor leads to release of a high concentrationof fluorine, which can, in turn, promote deterioration of the structureof the thin film product through etching. Etching creates pores in thethin film product and leads to a low density material having a highinternal surface area. The high surface area includes a highconcentration of surface defect states that detract from photovoltaicefficiency by promoting non-radiative recombination processes. The highsurface area is also reactive and promotes contamination of the materialwith environmental agents such as oxygen or nitrogen.

Management of the presence of fluorine provides a strategy forminimizing the tendency of fluorine to etch the product film. Thepresence of fluorine can be managed by controlling the timing offluorine introduction, the concentration of fluorine, and the form offluorine in the deposition environment. Hydrogen, for example, is onetool for controlling the form of fluorine because the simultaneouspresence of hydrogen and fluorine depletes the supply of active,dissociated fluorine through the formation of HF. By binding fluorinewith hydrogen, the overall supply of active fluorine can be regulatedand controlled to provide enough fluorine to promote favorable bondingconfigurations within the product thin film while avoiding etching tofacilitate formation of a dense, non-porous product film at highdeposition rates.

Example 1

In this example, selected compositional and optical absorptioncharacteristics of representative thin film materials comprisingamorphous silicon in accordance with the instant invention aredescribed. The materials are denominated Sample 547 and Sample 548 andwere prepared in a deposition system similar to that shown in FIG. 4that included a single microwave applicator with a single conduitpassing therethrough and a single supplemental inlet for delivering anon-energized supplemental material stream. The conduit was made fromsapphire and the substrate was positioned about 4 inches from theinterface of the conduit with the deposition chamber. A mixture of 1standard liter per minute of SiF₄ and 2 standard liters per minute ofargon was introduced to the conduit of the microwave applicator andactivated with microwave radiation at a frequency of 2.45 GHz and apower of 600 W. SiH₄ was introduced at a rate of 1 standard liter perminute to the deposition chamber through the supplemental delivery portin an electrically-neutral state. The energized stream exiting theconduit of the microwave applicator and the non-energized streamsupplied by the supplemental delivery port were directed to a substrateand a thin film product material was formed therefrom. The substrate wasmaintained at a temperature of about 400° C. and positioned on a mount.An electrical bias could be optionally provided to the mount. For Sample547, the substrate bias was maintained at ground and for Sample 548, thesubstrate was AC-biased at 100 kHz with a 60V peak-to-peak biasingsignal. The deposition rate of Samples 547 and 548 was ˜140 Å/s.

FIGS. 5 and 6 show the results of a SIMS (secondary ion massspectrometry) analysis of the chemical composition of Samples 547 and548, respectively. The SIMS profile is a measure of the concentration ofdifferent elements in the composition as a function of depth within thesample. FIG. 5 indicates that the composition of Sample 547 wasprimarily silicon and also included 6.8% hydrogen, 0.3% fluorine, 0.05%oxygen, and 0.0003% nitrogen. FIG. 6 indicates that the composition ofSample 548 was primarily silicon and also included 6.0% hydrogen, 0.24%fluorine, 0.03% oxygen, and 0.0003% nitrogen. The low level of oxygen inSamples 547 and 548 indicates that both materials are free fromatmospheric contamination.

FIGS. 7 and 8 show the optical absorption spectra of Samples 547 and548, respectively, and provide comparisons with the optical absorptionspectra of two reference materials. The figures show the dependence ofthe absorption coefficient α as a function of photon energy (expressedin units of eV).

In FIG. 7, trace 305 corresponds to the optical absorption spectrum ofSample 547. Trace 310 (labeled “NREL” and corresponding to comparativeSample NREL) shows the optical absorption spectrum of a high qualityreference sample of amorphous silicon prepared by a slow rate prior artdeposition process. Sample NREL was non-fluorinated and corresponds tothe current state of the art for thin film amorphous silicon materials.Trace 315 (labeled “236” and corresponding to comparative Sample 236)shows the optical absorption spectrum of a low quality reference sampleof fluorinated amorphous silicon prepared by a high deposition rateprocess. Trace 310 for Sample NREL and trace 315 for comparative Sample236 are repeated in FIG. 8. FIG. 8 further includes trace 320 for Sample548.

Characterization of comparative Sample 236 indicated that the materialwas porous with a pore volume fraction of ˜10% based on refractive indexmeasurements. SIMS data indicated that comparative Sample 236 had acomposition that included 2.8% hydrogen, 0.15% fluorine, 0.34% oxygen,and 0.0032% nitrogen. The much higher oxygen concentration forcomparative Sample 236 relative to Samples 547 and 548 is consistentwith its high porosity and greater susceptibility of environmentalcontamination.

Comparative Sample NREL, comparative Sample 236, Sample 547, and Sample548 all exhibited a pronounced increase in optical absorption at aphoton energy of about 1.5 eV. The sharp increase in absorptioncoefficient represents the onset of the transition from the valence bandto the conduction band for each of the samples and the similarity of thephoton energy at which the transition occurs in each of the materialsindicates that the materials have similar bandgaps. The energy of thebandgap indicates that Samples 547 and 548 are predominantly amorphousphase materials.

The optical absorption spectrum of a semiconductor material at energiesbelow the bandgap provides a measure of the quality of the material. Inthe absence of defects (structural, electronic, or compositional), asemiconductor material should exhibit no absorption at energies belowthe bandgap. When defects are present, however, electronic states canform in the bandgap. These states can participate in optical transitionsbetween each other or with either or both of the valence band andconduction band to provide optical absorption features at energies belowthe bandgap energy. The presence of midgap defects states is undesirablefrom a performance perspective because they typically serve asrecombination or trapping centers that reduce photovoltaic efficiency.The intensity of optical absorption in the below bandgap portion of thespectrum is a measure of the density of defect states in the bandgap.Low optical absorption at below bandgap energies signifies a low densityof defect states and a higher quality product material.

A comparison of the optical absorption spectra shown in FIGS. 7 and 8indicates that comparative Sample NREL shows only weak absorption in thebelow bandgap spectral region (photon energies of less than ˜1.5 eV),while comparative Sample 236 exhibits pronounced absorption in the belowbandgap spectral region. These results indicate that comparative SampleNREL has a low concentration of defect states, while comparative Sample236 includes a high concentration of defect states.

The results show that Samples 547 and 548 exhibit significantly lessabsorption in the below bandgap spectral region than comparative Sample236. This is an indication that Samples 547 and 548 possess a much lowerconcentration of defects and a much lower density of states thancomparative Sample 236. The defect concentrations and density of statesof Samples 547 and 548 are only slightly greater than that ofcomparative Sample NREL.

The results indicate that the instant invention provides asilicon-containing amorphous semiconductor material at high depositionrate that has a defect concentration comparable to the state of the artfound in low deposition rate amorphous silicon. Relative to other highdeposition rate processes, the instant invention provides a denser, lessporous material that has a much lower concentration of midgap defects.

FIG. 9 shows the evolution of the optical absorption spectrum of Sample547 with time upon continuous exposure to incident electromagneticradiation. The optical absorption spectrum of Sample 547 at a particularlocation was measured before exposure to incident radiation. Sample 547was then subjected to continuous exposure to incident radiation and theoptical absorption spectrum at the same location was measured at varioustimes. Sample 547 was first exposed to radiation that simulates thesolar spectrum (AM-1) for 14 hours and the optical absorption spectrumwas measured. Sample 547 was next exposed to the sun for an additional12 hours and the optical absorption spectrum was measured again.Finally, Sample 547 was exposed to the AM-1 simulated solar spectrum fora further 59 hours (for a total 85-hour exposure time) and the opticalabsorption spectrum was measured again. FIG. 9 presents the results ofthe measurements and shows that the optical absorption spectrum ofSample 547 was virtually unchanged upon exposure to incident radiation.Constancy of the optical absorption spectrum indicates that density ofstates of Sample 547 did not increase during the period of exposure anddemonstrates that Sample 547 is essentially free from degradation due tothe Staebler-Wronski effect.

In one embodiment, the thin film material of the instant invention isfree from Staebler-Wronski degradation after exposure to the solarspectrum for at least 14 hours. In another embodiment, the thin filmmaterial of the instant invention is free from Staebler-Wronskidegradation after exposure to the solar spectrum for at least 26 hours.In a further embodiment, the thin film material of the instant inventionis free from Staebler-Wronski degradation after exposure to the solarspectrum for at least 85 hours.

FIG. 10 compares the effect of solar radiation on Sample 547 with theeffect of solar radiation on the NREL sample of conventional sample ofamorphous silicon referred to in FIG. 7. Sample 547 is referred to asthe “Ovshinsky Solar” sample and the NREL sample is referred to as“Standard a-Si” in FIG. 10. As indicated in FIG. 9, Sample 547 exhibitedlittle or no Staebler-Wronski degradation. The sample of conventionalamorphous silicon, however, exhibited pronounced degradation uponexposure to solar radiation. Before exposure to solar radiation, Sample547 exhibited slightly greater absorption at below bandgap energies thanconventional amorphous silicon (See FIG. 7). The density of states inthe bandgap for Sample 547 was therefore slightly higher than thedensity of states in the bandgap for conventional amorphous siliconbefore exposure to solar radiation. After exposure to solar radiation,however, conventional amorphous silicon exhibited significantly greaterabsorption at below bandgap energies than Sample 547. Based on the datashown in FIG. 10, it is estimated that the density of states in Sample547 after exposure to solar radiation is less by a factor of five thanthe density of states in conventional amorphous silicon. Unlikeconventional amorphous silicon, materials prepared in accordance withthe instant invention exhibit stable optical properties withoutphotogeneration of midgap or near edge defect states when exposed tosolar radiation. Convention amorphous silicon, in contrast, exhibitsappreciable Staebler-Wronski degradation.

The higher density of states resulting from the photodegradation ofconventional amorphous silicon affects not only optical properties, butalso electrical properties. The higher density of defect states inconventional amorphous silicon function as carrier traps that reducecarrier mobility and conductivity. Mobility and conductivity ofmaterials prepared in accordance with the instant invention remainstable upon exposure to electromagnetic radiation. The low density ofstates and superior electrical properties of the instant materialsindicate that the instant materials have utility beyond photovoltaicsinto a broader array of electronic applications. The instant materials,for example, provide excellent prospects for transistors and diodesbased on amorphous silicon. The instant invention motivates a truesilicon-based thin film electronics technology.

This example demonstrates the remarkable result that asilicon-containing photovoltaic material that exhibits a low density ofstates and essentially no Staebler-Wronski degradation can be depositedat a rate ˜140 Å/s.

Example 2

In this example, the effect of process gas ratio on the deposition rateand photoconductivity of representative materials comprising amorphoussilicon in accordance with the instant invention is described. Thesamples were prepared using the deposition system described in Example 1hereinabove. A mixture of 1 standard liter per minute of SiF₄ and 2standard liters per minute of argon was introduced to the conduit of themicrowave applicator and activated with microwave radiation at afrequency of 2.45 GHz and a power of 600 W. Instead of SiH₄, however,disilane (Si₂H₆) was introduced to the deposition chamber through thesupplemental delivery port and delivered as an electrically-neutralmaterial stream. The flow rate of disilane was systematically adjustedto provide a series of samples for which the ratio of the flow rate ofdisilane to the ratio of the flow rate of SiF₄ ranged from 0.3 to 2.0.The energized stream of SiF₄ and argon exiting the conduit of themicrowave applicator and the non-energized stream of disilane suppliedby the supplemental delivery port were directed to a substrate and athin film product material was formed therefrom. The substrate wasmaintained at a temperature of 400° C. and positioned on a groundedmount.

The bandgap, deposition rate, and μτ product were measured for eachsample. The bandgap generally increased with increasing Si₂H₆/SiF₄ flowrate ratio and varied from 1.59 eV at a flow rate ratio of 0.3 to 1.64eV at a flow rate ratio of 2.0.μ and τ correspond to carrier mobilityand carrier lifetime, respectively, upon photoexcitation at a particularwavelength. The μτ product was measured at two above bandgapwavelengths: 565 nm and 660 nm. The μτ product correlates with thephotoconductivity of each sample. A higher value for the μτ productindicates better photoconductivity and a higher quality material.Deposition rate was determined from the thickness of the depositedmaterial and the time of deposition.

FIG. 11 shows the variation of the μτ product as a function of theSi₂H₆/SiF₄ flow rate ratio. The μτ product is expressed in units ofcm²/V. The results indicate that the μτ product was higher uponexcitation at 660 nm than upon excitation at 565 nm, but that the μτproduct at both excitation wavelengths was maximized at a Si₂H₆/SiF₄flow rate ratio of about 1.25.

FIG. 12 shows the variation of deposition rate with the Si₂H₆/SiF₄ flowrate ratio. The deposition rate increased from 93 Å/s at a Si₂H₆/SiF₄flow rate ratio of 0.3, peaked at 139 Å/s at a Si₂H₆/SiF₄ flow rateratio of 1.0 and gradually declined at Si₂H₆/SiF₄ flow rate ratios above1.0.

The data show that the highest values of the μτ product and the highestdeposition rates both occurred over the range of Si₂H₆/SiF₄ flow rateratio between 1.0 and 1.5. This example therefore demonstrates that thehighest quality samples are formed at the highest deposition rates.

Example 3

In this example, the effect of substrate temperature on the depositionrate and photoconductivity of representative materials comprisingamorphous silicon in accordance with the instant invention is described.The samples were prepared using the deposition system described inExample 1 hereinabove. A mixture of 1 standard liter per minute of SiF₄and 2 standard liters per minute of argon was introduced to the conduitof the microwave applicator and activated with microwave radiation at afrequency of 2.45 GHz and a power of 600 W. Instead of SiH₄, however,disilane (Si₂H₆) was introduced at a rate of 1 standard liter per minuteto the deposition chamber through the supplemental delivery port. Thedisilane was delivered in an electrically-neutral state. The Si₂H₆/SiF₄flow rate ratio was fixed at 1.0 in these experiments. The energizedstream of SiF₄ and argon exiting the conduit of the microwave applicatorand the non-energized stream of disilane supplied by the supplementaldelivery port were directed to a substrate and a thin film productmaterial was formed therefrom. The substrate was placed on a groundedmount. A series of samples was prepared for which the substratetemperature was varied between 300° C. and 495° C.

The bandgap, deposition rate, and μτ product were measured as describedin Example 2 hereinabove as a function of substrate temperature. Thebandgap was 1.78 eV at a substrate temperature of 300° C. and decreasedin an approximately linear manner to 1.52 eV at a substrate temperatureof 495° C. The deposition rate was relatively constant, but showed aslight decrease from about 150 Å/s for substrate temperatures of 375° C.or less to about 135-140 Å/s for substrates temperatures of 400° C. orhigher.

The variation of the μτ product as a function of substrate temperatureis shown in FIG. 13. The μτ product was measured using excitationwavelengths of 565 nm and 660 nm. The data indicate that the μτ productwas highest for substrate temperatures between 375° C. and 475° C. andwas lower for substrate temperatures above and below this range.

The index of refraction was also measured for this series of samples andshowed a progressive increase with increasing substrate temperature from3.02 at a substrate temperature of 300° C. to 3.59 at a substratetemperature of 495° C. The refractive index was above 3.5 for substratesat or above 425° C. The results indicate that the samples becameincreasingly dense and non-porous as the substrate temperature wasincreased. For comparison purposes, the refractive index of fullydensified amorphous silicon is about 3.6.

This example shows that the optimum substrate temperature for depositinga silicon-containing photovoltaic material from Si₂H₆ and SiF₄ at a flowrate ratio of 1.0 is between 375° C. and 475° C. The results show thathigh quality samples (as judged by the μτ product and refractive index)can be prepared at extremely high deposition rates using the principlesof atomic engineering annunciated herein.

The foregoing discussion demonstrates that the instant invention permitssimultaneous realization of the benefits of hydrogen and fluorine. Asnoted hereinabove, both hydrogen and fluorine passivate dangling bondsand relieve bond strain. Since the bond strengths of hydrogen andfluorine with silicon differ, however, hydrogen and fluorine may exhibita preferential effectiveness for remediating energetically distinctdefects within the spectrum of defects known to exist in the variousstructural forms of silicon (amorphous, intermediate range order,nanocrystalline, and microcrystalline). As a result, material ofparticularly high quality can be expected through the combined effectsof fluorine and hydrogen. With this invention, such material can beformed continuously at high deposition rates in a microwave plasmadeposition process for the first time and unique bonding configurationscan be achieved that minimize the density of states and suppress theStaebler-Wronski effect through careful control of the relative amountsof hydrogen and fluorine.

One objective is to maximize the amount of fluorine in the productmaterial, but to do so from a deposition environment in which theconcentration of active fluorine remains below the thresholdconcentration at which etching occurs. Fluorine in excess of thethreshold concentration may be present in the deposition environmentprovided it is inactive. The presence of inactive fluorine isadvantageous because it can be converted to active fluorine through, forexample, a supplemental energy source in the deposition chamber asdescribed hereinabove to serve as a local source of active fluorine tocompensate for the depletion of fluorine in the deposition environmentas it becomes incorporated into the product film.

In one embodiment, the concentration of fluorine in a silicon-containingphotovoltaic material is between 0.1% and 7%. In another embodiment, theconcentration of fluorine in a silicon-containing photovoltaic materialis between 0.2% and 5%. In a further embodiment, the concentration offluorine in a silicon-containing photovoltaic material is between 0.5%and 4%. In one embodiment, the concentration of fluorine is as indicatedabove and the concentration of hydrogen is less than 15%. In anotherembodiment, the concentration of fluorine is as indicated above and theconcentration of hydrogen is less than 10%. In a further embodiment, theconcentration of fluorine is as indicated above and the concentration ofhydrogen is less than 7%.

Deposition species may also include precursors designed to achieven-type or p-type doping. Doping precursors include gas phase compoundsof boron (e.g. boranes, organoboranes, fluoroboranes), phosphorous (e.g.phosphine, organophosphines, or fluorophosphines), arsenic (e.g. arsineor organoarsines), and SF₆. One or more deposition or doping precursorsmay be introduced to one or more conduits individually, sequentially, orin combination.

The instant microwave deposition apparatus and method may be used toform amorphous, nanocrystalline, microcrystalline, or polycrystallinematerials, or combinations thereof as a single layer or in a multiplelayer structure. In one embodiment, the instant deposition apparatusincludes a plurality of deposition chambers, where at least one of thedeposition chambers is equipped with a remote plasma source having thecapabilities described hereinabove. The different chambers may formmaterials of different composition, different doping, and/or differentcrystallographic form (amorphous, nanocrystalline, microcrystalline, orpolycrystalline).

The instant deposition process provides thin film materials havingcompositions within the scope of the instant invention at highdeposition rates with a low density of defect states and little or noStaebler-Wronski effect. In one embodiment, the thin film depositionrate is at least 20 Å/s. In another embodiment, the thin film depositionrate is at least 50 Å/s. In still another embodiment, the thin filmdeposition rate is at least 100 Å/s. In a further embodiment, the thinfilm deposition rate is at least 150 Å/s. In one embodiment, thin filmmaterials formed at the foregoing deposition rates exhibit essentiallyno Staebler-Wronski degradation after exposure to an AM-1 solar spectrumfor at least 14 hours. In another embodiment, thin film materials formedat the foregoing deposition rates exhibit essentially noStaebler-Wronski degradation after exposure to an AM-1 solar spectrumfor at least 26 hours. In still another embodiment, thin film materialsformed at the foregoing deposition rates exhibit essentially noStaebler-Wronski degradation after exposure to an AM-1 solar spectrumfor at least 85 hours.

In one embodiment, the temperature of the substrate is between 300° C.and 500° C. In another embodiment, the temperature of the substrate isbetween 325° C. and 475° C. In one embodiment, the temperature of thesubstrate is between 350° C. and 450° C.

In one embodiment, the substrate is electrically grounded. In anotherembodiment, the substrate is electrically biased. In a furtherembodiment, the electrical bias is an AC bias.

In one embodiment, the molar or volumetric flow rate of disilane to SiF₄is between 0.3 and 2.0. In another embodiment, the molar or volumetricflow rate of disilane to SiF₄ is between 0.5 and 1.75. In still anotherembodiment, the molar or volumetric flow rate of disilane to SiF₄ isbetween 0.75 and 1.5. Since disilane includes 2 moles of silicon permole of precursor, the flow rate ratios will be doubled when usingsilane as a precursor in conjunction with SiF₄. In one embodiment, themolar or volumetric flow rate of silane to SiF₄ is between 0.6 and 4.0.In another embodiment, the molar or volumetric flow rate of silane toSiF₄ is between 1.0 and 3.5. In still another embodiment, the molar orvolumetric flow rate of disilane to SiF₄ is between 1.5 and 3.0.

The instant deposition apparatus is adapted to deposit one or more thinfilm materials on a continuous web or other moving substrate. In oneembodiment, a continuous web substrate or other moving substrate isadvanced through each of a plurality of deposition chambers and asequence of layers is formed on the moving substrate. The individualdeposition chambers within the plurality are operatively interconnectedand environmentally protected to prevent intermixing of the depositionspecies introduced into the individual chambers. Gas gates, for example,may be placed between the chambers to prevent intermixing. A variety ofmultiple layer or stacked cell device configurations may be obtained.

FIG. 14 shows a portion of a deposition system in accordance with theinstant invention that includes a continuous web substrate. Thedeposition system includes deposition chamber 260 equipped withcontinuous web substrate 230. Continuous web substrate 230 is in motionduring deposition and is delivered to deposition chamber 260 by payoutroller 265 and received by take up roller 270 after deposition of thinfilm material 275. Continuous web substrate 230 enters and exitsdeposition chamber 260 through isolation devices 280. Isolation devices280 may be, for example, gas gates. Deposition chamber 260 receivesstreams 250 and 255 containing energized or activated deposition speciesfrom separate conduits (not shown) of a microwave applicator (not shown)as described hereinabove. Streams 250 and 255 enter deposition chamber260 through inlets 220 and 225. Inlets 220 and 225 may correspond tooutlets of conduits that pass through a microwave applicator. Streams250 and 255 are directed to the surface of substrate 230 and combine orother react or interact to form thin film material 275. Depositionchamber may optionally be equipped with independent means for generatinga plasma to further energize or activate streams 250 and 255. Additionaldeposition chambers may be operatively connected to deposition chamber260 to permit formation of a multilayer thin film structure or device.

One important multilayer photovoltaic device is the triple junctionsolar cell, which includes a series of three stacked n-i-p devices withgraded bandgaps on a common substrate. The graded bandgap structureprovides more efficient collection of the solar spectrum. In making ann-i-p photovoltaic device, a first chamber is dedicated to thedeposition of a layer of an n-type semiconductor material, a secondchamber is dedicated to the deposition of a layer of substantiallyintrinsic (i-type) semiconductor material, and a third chamber isdedicated to the deposition of a layer of a p-type semiconductormaterial. In one embodiment, the intrinsic semiconductor layer is anamorphous semiconductor that includes silicon, germanium, or an alloy ofsilicon and germanium. The n-type and p-type layers may bemicrocrystalline or nanocrystalline forms of silicon, germanium, or analloy of silicon and germanium. The process can be repeated by expandingthe deposition apparatus to include additional chambers to achieveadditional n-type, p-type, and/or i-type layers in the structure. Atriple cell structure, for example, can be achieved by extending theapparatus to include six additional chambers to form a second and thirdn-i-p structure on the web. Tandem devices and devices that include p-njunctions are also within the scope of the instant invention.

Bandgap grading of multiple junction device structures may be achievedby modifying the composition of the intrinsic (i-type) layer in theseparate n-i-p subunits. In one embodiment, the highest bandgap in thetriple junction cell results from incorporation of amorphous silicon asthe intrinsic layer in one of the n-i-p structures. Alloying of siliconwith germanium to make amorphous silicon-germanium alloys leads to areduction in bandgap. The second and third n-i-p structures of a triplejunction cell may include intrinsic layers comprising SiGe alloys havingdiffering proportions of silicon and germanium. In this way, each of thethree intrinsic layers of a triple cell device has a distinct bandgapand each bandgap can be optimized to absorb a particular portion of theincident solar or electromagnetic radiation.

In one device configuration, the incident radiation first encounters ann-i-p structure that includes an amorphous silicon intrinsic layer. Theamorphous silicon intrinsic layer absorbs the shorter wavelengthfraction of the incident radiation (e.g. shorter wavelength visible andultraviolet wavelengths) and transmits the longer wavelength fraction(e.g. middle and longer wavelength visible and infrared wavelengths).The longer wavelength fraction next encounters a second intrinsic layerthat includes a silicon-germanium alloy having a relatively lowergermanium content. The second intrinsic layer absorbs the shorterwavelength portion (e.g. middle wavelength visible portion) of thelonger wavelength fraction transmitted by the amorphous siliconintrinsic layer and transmits the longer wavelength portion (e.g. longwavelength visible and infrared wavelengths) to a third intrinsic layerhaving an intrinsic layer that includes a silicon-germanium alloy with arelatively higher germanium content. By grading the bandgaps of theintrinsic layers, more efficient absorption of the incident radiationoccurs and better conversion efficiency is achieved.

In addition to compositional variation, bandgap modification may also beachieved through control of the microstructure of the intrinsic layer.Polycrystalline silicon, for example, has a different bandgap thanamorphous silicon and multilayer stacks of various structural phases maybe formed with the instant continuous web apparatus. The nanocrystallineand intermediate range order forms of silicon can provide bandgapsbetween the bandgap of crystalline silicon and the bandgap of amorphoussilicon.

Another important multilayer structure is the p-n junction. Inconventional amorphous silicon or hydrogenated amorphous silicon, thehole mobility is too low to permit efficient operation of a p-njunction. The low hole mobility is a consequence of a high defectdensity that leads to efficient trapping of charge carriers before theycan be withdrawn as external current. To compensate for carriertrapping, an i-layer is often included in the structure. With thematerial prepared by the instant invention, the defect concentration inn-type or p-type material is greatly reduced and efficient p-n junctionscan be formed from silicon, germanium, and silicon-germanium alloys.Alternatively, p-i-n structure can be formed in which the i-layerthickness necessary for efficient charge separation is much smaller thatis required for current devices.

Those skilled in the art will appreciate that the methods and designsdescribed above have additional applications and that the relevantapplications are not limited to the illustrative examples describedherein. The present invention may be embodied in other specific formswithout departing from the essential characteristics or principles asdescribed herein. The embodiments described above are to be consideredin all respects as illustrative only and not restrictive in any mannerupon the scope and practice of the invention. It is the followingclaims, including all equivalents, which define the true scope of theinstant invention.

1. A deposition process comprising: providing a first precursor, saidfirst precursor comprising silicon; subjecting said first precursor tomicrowave radiation, said microwave radiation exciting said firstprecursor to form an energized deposition medium; directing saidenergized deposition medium toward a substrate, said energizeddeposition medium having a first state remote from said substrate and asecond state adjacent said substrate, said first state comprising aplasma, said second state lacking a plasma and being ionized; providinga second precursor, said second precursor being in an electricallyneutral state; and directing said second precursor to said substrate,said second precursor combining with said energized deposition medium toform a thin film material on said substrate.
 2. The process of claim 1,wherein said first precursor further comprises fluorine.
 3. The processof claim 1, wherein said first precursor comprises SiF₄.
 4. The processof claim 1, wherein said first precursor lacks hydrogen.
 5. The processof claim 1, wherein said second state is a lower energy state than saidfirst state.
 6. The process of claim 1, wherein said substrate iselectrically grounded.
 7. The process of claim 1, wherein said substrateis electrically biased.
 8. The process of claim 7, wherein said bias isan AC bias.
 9. The process of claim 1, wherein the temperature of saidsubstrate is between 300° C. and 500° C.
 10. The process of claim 1,wherein the temperature of said substrate is between 325° C. and 475° C.11. The process of claim 1, wherein the temperature of said substrate isbetween 350° C. and 450° C.
 12. The process of claim 1, wherein saidsecond precursor comprises silicon.
 13. The process of claim 12, whereinsaid second precursor further comprises hydrogen.
 14. The process ofclaim 13, wherein said second precursor comprises silane.
 15. Theprocess of claim 13, wherein said second precursor comprises disilane.16. The process of claim 13, wherein the molar flow ratio of said secondprecursor to said first precursor is between 0.3 and 2.0.
 17. Theprocess of claim 13, wherein the molar flow ratio of said secondprecursor to said first precursor is between 0.5 and 1.75.
 18. Theprocess of claim 13, wherein the molar flow ratio of said secondprecursor to said first precursor is between 0.75 and 1.5.
 19. Theprocess of claim 13, wherein the molar flow ratio of said secondprecursor to said first precursor is between 0.6 and 4.0.
 20. Theprocess of claim 13, wherein the molar flow ratio of said secondprecursor to said first precursor is between 1.0 and 3.5.
 21. Theprocess of claim 13, wherein the molar flow ratio of said secondprecursor to said first precursor is between 1.5 and
 3. 22. The processof claim 1, wherein said thin film material comprises amorphous silicon.23. The process of claim 22, wherein said thin film material furthercomprises nanocrystalline, microcrystalline, or polycrystalline silicon.24. The deposition process of claim 22, wherein said thin film materialis formed at a deposition rate of at least 20 Å/s.
 25. The depositionprocess of claim 22, wherein said thin film material is formed at adeposition rate of at least 50 Å/s.
 26. The deposition process of claim22, wherein said thin film material is formed at a deposition rate of atleast 100 Å/s.
 27. The deposition process of claim 22, wherein said thinfilm material is formed at a deposition rate of at least 150 Å/s. 28.The deposition process of claim 1, wherein at least one of said firstprecursor and said second precursor comprises fluorine and said thinfilm material exhibits essentially no Staebler-Wronski degradation uponexposure to an AM-1 solar spectrum for at least 14 hours.
 29. Thedeposition process of claim 28, wherein said thin film material exhibitsessentially no Staebler-Wronski degradation upon exposure to an AM-1solar spectrum for at least 26 hours.
 30. The deposition process ofclaim 28, wherein said thin film material exhibits essentially noStaebler-Wronski degradation upon exposure to an AM-1 solar spectrum forat least 85 hours.
 31. The deposition process of claim 28, wherein saidthin film material is formed at a deposition rate of at least 20 Å/s.32. The deposition process of claim 28, wherein said thin film materialis formed at a deposition rate of at least 50 Å/s.
 33. The depositionprocess of claim 28, wherein said thin film material is formed at adeposition rate of at least 100 Å/s.